Re: [PATCH v2 1/2] media: videobuf2: Fix dmabuf cache sync/flush in dma-contig

[Mikhail Rudenko <mike.rudenko@gmail.com>]

Hi Tomasz,

and thanks for your review!

On 2025-01-27 at 20:18 +09, Tomasz Figa <tfiga@chromium.org> wrote:

> Hi Mikhail,
>
> On Thu, Jan 16, 2025 at 2:25 AM Mikhail Rudenko <mike.rudenko@gmail.com> wrote:
>>
>> When support for V4L2_FLAG_MEMORY_NON_CONSISTENT was removed in
>> commit 129134e5415d ("media: media/v4l2: remove
>> V4L2_FLAG_MEMORY_NON_CONSISTENT flag"),
>> vb2_dc_dmabuf_ops_{begin,end}_cpu_access() functions were made
>> no-ops. Later, when support for V4L2_MEMORY_FLAG_NON_COHERENT was
>> introduced in commit c0acf9cfeee0 ("media: videobuf2: handle
>> V4L2_MEMORY_FLAG_NON_COHERENT flag"), the above functions remained
>> no-ops, making cache maintenance for non-coherent dmabufs allocated by
>> dma-contig impossible.
>>
>> Fix this by reintroducing dma_sync_sg_for_{cpu,device} calls to
>> vb2_dc_dmabuf_ops_{begin,end}_cpu_access() functions for non-coherent
>> buffers.
>>
>> Fixes: c0acf9cfeee0 ("media: videobuf2: handle V4L2_MEMORY_FLAG_NON_COHERENT flag")
>> Cc: stable@vger.kernel.org
>> Signed-off-by: Mikhail Rudenko <mike.rudenko@gmail.com>
>> ---
>>  drivers/media/common/videobuf2/videobuf2-dma-contig.c | 14 ++++++++++++++
>>  1 file changed, 14 insertions(+)
>>
>
> Thanks a lot for the patch!
> Sorry, for the delay. Ended up being sick with some nasty cold that
> took quite a while to recover.
> Please take a look at my comments inline.
>
>> diff --git a/drivers/media/common/videobuf2/videobuf2-dma-contig.c b/drivers/media/common/videobuf2/videobuf2-dma-contig.c
>> index bb0b7fa67b539aa73ad5ccf3c3bc318e26f8a4cb..889d6c11e15ab5cd4b4c317e865f1fef92df4b66 100644
>> --- a/drivers/media/common/videobuf2/videobuf2-dma-contig.c
>> +++ b/drivers/media/common/videobuf2/videobuf2-dma-contig.c
>> @@ -427,6 +427,13 @@ static int
>>  vb2_dc_dmabuf_ops_begin_cpu_access(struct dma_buf *dbuf,
>>                                    enum dma_data_direction direction)
>>  {
>> +       struct vb2_dc_buf *buf = dbuf->priv;
>> +       struct sg_table *sgt = buf->dma_sgt;
>> +
>> +       if (!buf->non_coherent_mem || buf->vb->skip_cache_sync_on_finish)
>
> skip_cache_sync_on_finish shouldn't apply to this function, because
> the buffer was shared with an external DMA-buf importer and we don't
> know in what state it is at this point.
>

Ack, will fix in v3.

>> +               return 0;
>> +
>
> We should also take care of the kernel mapping if it exists, because
> on some platforms it may not be coherent with the userspace one -
> using flush_kernel_vmap_range(). Please check how
> vb2_dc_prepare()/vb2_dc_finish() do it.
>

Thanks for the pointer, will do so in v3.

>> +       dma_sync_sg_for_cpu(buf->dev, sgt->sgl, sgt->nents, buf->dma_dir);
>
> We can use the dma_sync_sgtable_*() variant here so we can just pass
> the entire sgt to it.
>

Will do so.

>>         return 0;
>>  }
>>
>> @@ -434,6 +441,13 @@ static int
>>  vb2_dc_dmabuf_ops_end_cpu_access(struct dma_buf *dbuf,
>>                                  enum dma_data_direction direction)
>>  {
>> +       struct vb2_dc_buf *buf = dbuf->priv;
>> +       struct sg_table *sgt = buf->dma_sgt;
>> +
>> +       if (!buf->non_coherent_mem || buf->vb->skip_cache_sync_on_prepare)
>> +               return 0;
>> +
>> +       dma_sync_sg_for_device(buf->dev, sgt->sgl, sgt->nents, buf->dma_dir);
>>         return 0;
>
> Overall the same comments here as for
> vb2_dc_dmabuf_ops_begin_cpu_access() +/- flush would change to
> invalidate.

Ack.

> Best regards,
> Tomasz


--
Best regards,
Mikhail Rudenko